Mammalian target of rapamycin complex 1 (mTORC1) is a critical regulator of cell growth and metabolism that integrates a variety of signals, both intracellular and extracellular, with the protein translation machinery. Signals from growth factors and energy stores are relayed to mTORC1 through the tuberous sclerosis complex proteins 1 (TSC1) and 2 (TSC2), which form a complex (TSC1/TSC2) with tumor suppressor function. We have discovered that mTORC1 regulation by oxygen levels also requires the TSC1/TSC2 complex. Failure to inhibit mTORC1 in TSC1/TSC2-deficient cells in response to hypoxia results in abnormal cell proliferation and might contribute to tumor growth. Recently, we established that the gene regulated in development and DNA damage 1 (REDD1), a gene of hitherto unknown function, was necessary for mTORC1 inhibition by hypoxia. REDD1 is transcriptionally induced in response to hypoxia and REDD1 overexpression is sufficient to inhibit mTORC1. REDD1 encodes a conserved 25 kDa protein with no recognizable structural or functional domains and no homology to other proteins of known function. Herein, data is presented showing that REDD1 forms a complex that contains a single REDD1 monomer and experiments are proposed to evaluate the role of the complex in REDD1 signaling. Structure-function analyses have revealed the existence of two domains in REDD1 that are required for function, and experiments are presented to test how these domains act. In addition, experiments are outlined to assess whether the REDD1 complex regulates mTORC1 directly, or through TSC1/TSC2. Preliminary data is also presented characterizing the subcellular localization of REDD1 and experiments are proposed to evaluate the mechanism that governs REDD1 subcellular distribution and its functional significance. Finally, a novel mouse strain has been generated and experiments are outlined to characterize the regulation and mechanism of REDD1 action in hypoxia signaling in the mouse. mTORC1 is deregulated in many pathological conditions and understanding how mTORC1 is regulated by hypoxia and REDD1 might have implications for human health. PUBLIC HEALTH RELEVANCE: This project seeks to understand the mechanism whereby cells adapt to changes in their environment. In particular we are interested in understanding how cells adapt to low oxygen levels. This process involves the inhibition of a cellular protein complex called mammalian target of rapamycin complex 1 (mTORC1) and this complex is implicated in multiple pathological processes. mTORC1 inhibitors have, in fact, been approved by the FDA for (1) the treatment of cancer, (2) to prevent transplant rejection, and (3) to prevent coronary artery stent occlusions. Thus, understanding how mTORC1 is regulated has profound implications for human health.